The present invention relates to an optical element assembly for use, for example, in the fields of optical communications, which permits highly accurate positioning of a microminiature optical element, such as a photonic crystal optical element, and an optical part, such as an optical fiber, with respect to each other. The invention also pertains to a method for the manufacture of such an optical element assembly.
In the fields of optical communications, a lot of attention has recently been given to artificial crystal materials (photonic crystals) whose refractive index changes with a period on the order of light wavelength. With the use of such photonic crystals, it is expected that advanced light-controlled functional elements which were impossible to realize in the past, such as a low-loss macrobend waveguide, a low-threshold laser and a wavelength division multiplexing element, can be fabricated in sizes of 100 μm×100 μm or smaller. With rapid progress in microfabrication technology, there have recently been made reports on prototypes of ultraminiature optical elements constructed from three- and two-dimensional photonic crystals for operation in a wavelength band for optical communications.
Slab-type photonic crystal optical waveguides are typical representatives of optical elements using the two-dimensional photonic crystals. The slab-type photonic crystals roughly fall into those of a structure in which a thin-film layer of a high refractive index material is perforated with two-dimensionally periodic arrays of air holes and those of a structure in which a thin-film layer of a low refractive index material has formed therein a two-dimensionally periodic arrays of rods of a high refractive index material. In both of the photonic crystals a photonic band gap develops when particular periodicity conditions are satisfied. By introducing air holes or an area without rods, that is, crystal defects into the periodic structure, an optical waveguide is formed which permits the propagation therethrough of light; namely, a line defect of missing holes or rods is defined to form the optical waveguide. Accordingly, the two-dimensional photonic crystals are considered as promising for realizing low-loss, ultraminiature optical waveguides.
However, since light inlet and outlet ports of such a slab type photonic crystal optical waveguide are approximately 1 μm or less in size (in thickness) that is smaller than the core diameter (about 4 to 10 μm in single mode) of an optical fiber, high-precision positioning of the waveguide and the optical fiber is difficult to achieve—this constitutes an obstacle to high-efficiency input of light to the waveguide. In the case of the optical waveguide of the photonic crystal having air holes periodically arranged in the high refractive index material layer, since light is launched into the layer from air, reflectivity on the plane of incidence is high, giving rise to a heavy coupling loss.
Particularly advantageously the optical elements using photonic crystals are ultraminiature as referred to above, but substantially no consideration has been paid to simple and effective methods for coupling the photonic crystals to optical parts which are absolutely necessary for using them as optical elements, such as an optical fiber for signal transmission, a microminiature lens and so forth, and a method for keeping the coupling loss down.
For high-precision optical coupling of an optical element and an optical fiber or similar optical part, there have widely been used methods using a V-grooved structure in a single crystal silicon substrate. A V-groove can easily be formed in single crystal silicon by what is called single crystal silicon anisotropic wet etching (hereinafter referred to simply as anisotropic etching) that makes use of an anisotropic etching characteristic of single crystal silicon based on its crystal orientation. A simple example of such a method is disclosed, for example, in Japanese Patent Application Kokai Publication Gazette No. 281360/97 (published Oct. 31, 1997, hereinafter referred to as document 1). According to this method, one end portion of an optical fiber is fixed in a V-groove of a single crystal silicon substrate and a semiconductor laser or similar optical element is directly mounted on the non-grooved substrate surface on the side opposite the optical element in alignment therewith.
Another method is proposed, for example, in Japanese Patent Application Kokai Publication Gazette No. 2002-14258 (published Jan. 18, 2002, hereinafter referred to as document 2).
With reference to FIG. 1, the proposed method will be described below in brief. A single crystal silicon wafer, which has its principal plane formed by a crystal plane (100) or (101), is subjected to photolithography and anisotropic etching in this order to form rows and columns of square concavities of the same size and each having the same shape as the outside shape of an inverted frustum of a pyramid, and the silicon wafer is diced crosswise centrally of each square concavity into a large number of optical element carriers 1. On the bottom of such an optical element carrier 1 there is formed a cross-shaped convexity 2 resulting from the formation of each square concavity by anisotropic etching. Each side surface of the cross-shaped convexity 2 is tapered by anisotropic etching with high precision, that is, the cross-shaped convexity becomes narrower toward its protruding end (toward the principal plane or the top surface of the wafer). An optical semiconductor element 3 is mounted on one side of the optical element carrier 1 (on the thick portion of the wafer).
A single crystal silicon platform 4 has formed in its top surface by anisotropic etching cross-shaped V-groove 5 and a smaller V-groove 6 in alignment with the horizontal V-groove. The V-grooves 5 and 6 are separated by a square-sectioned groove 7 that extends at right angles to but communicate with them. Incidentally, such platforms are also simultaneously produced in quantities using a single crystal silicon wafer in the same manner as in the case of the optical element carrier 1. The optical element carrier 1 is mounted on the platform 4 with the cross-shaped convexity 2 of the former fitted in the cross-shaped V-groove 5 of the latter, and an optical fiber 8 is positioned in the V-groove 6. The cross-shaped convexity 2 and the cross-shaped V-groove 5 are high in dimensional accuracy, and through utilization of their mechanical accuracy, the optical element carrier 1 can be mounted on the platform 4 with high accuracy. This ensures satisfactory optical coupling between the optical semiconductor element 3 mounted on the carrier 1 and the core of the optical fiber 8 disposed in the V-groove 6.
According to the method of document 1, however, the optical element is fixedly mounted to the V-grooved substrate as by flip chip bonding while visually identifying a marker attached to the substrate. In the method of document 2, too, the optical element 3 is similarly mounted on the optical element carrier 1. Accordingly, the positioning accuracy is dependent on the accuracy of bonding and lower than in the case of engagement between a V-groove and concave-convex structures (convexity and groove) formed by anisotropic etching.
On the other band, a solution to this problem is set forth, for example, in Japanese Patent Application Kokai Publication Gazette No. 313756/96 (published Nov. 29, 1996, hereinafter referred to as document 3). According to the proposed method, a V-groove is cut in one surface area of a single crystal silicon and an optical waveguide is formed in alignment with the V-groove by coating a film directly on the non-grooved surface area of the substrate, thereby achieving positioning with accuracy of microfabrication.
It is difficult to mount such a microminiature optical element as the photonic crystal element by the methods set forth in documents 1 and 2, and even if it could be mounted, it gives rise to the above-mentioned problem that the positioning accuracy depends on the bonding accuracy. In the case of forming such a microminiature optical element as the photonic crystal element directly on the V-grooved substrate, the microminiature optical element takes up so small an amount of the V-grooved substrate surface that the manufacturing costs inevitably rise. In other words, since this method cannot utilize the technique of simultaneously forming a large number of microminiature optical elements on a substrate or wafer and severing them into individual elements, it is impossible to reduce the manufacturing costs through mass-production, failing to take advantage of microminiature optical elements.